I've got a very similar setup as you have:
- 180 litre tank
- 11 Panda corries
- 17 Cardinal Tetras
- 4 big platy & some little ones (2 Platy Hawii, 2 'mixed')
- Cherry shrimp
I chose the platy's because they have similar requirements as the other fish (e.g. regarding temperature), also because they're easy to breed, and they will basically swim in all layers of the tank (top, middle, bottom).
Other fish I have considered are guppies, but decided not to take them because they breed too fast.
Even with the platy's, I think eventually I will have to sell some, but so far I don't think my tank is overstocked.
Another fish you can consider is a betta. If you want them to breed you should get a male and 2 or 3 females.
Before the platys, I also had one. But after it died (from old age), I decided not to get one again, because I've got a bit too much current in my tank.
One thing you have to keep in mind is that fish eat shrimp.
If you only have 4 adults now, it's possible that they won't be able to keep the population stable or get it growing, and that you won't have any left in 6 months.
But it does sound like they have quit some hiding places, so it might work.
I started with 10-15 shrimp about a year ago, and the population seems stable/growing (I definitely have more than 15 now). But I also have a lot of hiding places.